Hi Neil, On 24 August 2017 at 06:07, NeilBrown <neilb@xxxxxxxx> wrote: > On Wed, Aug 23 2017, Ian Kent wrote: > >> >> That inconsistency has bothered me for quite a while now. >> >> It was carried over from the autofs module behavior when automounting >> support was added to the VFS. What's worse is it prevents the use of >> the AT_NO_AUTOMOUNT flag from working properly with fstatat(2) and with >> statx(). >> >> There is some risk in changing that so it does work but it really does >> need to work to enable userspace to not trigger an automount by using >> this flag. >> >> So that's (hopefully) going to change soonish, see: >> http://ozlabs.org/~akpm/mmotm/broken-out/autofs-fix-at_no_automount-not-being-honored.patch >> >> The result should be that stat family calls don't trigger automounts except >> for fstatat(2) and statx() which will require the AT_NO_AUTOMOUNT flag. >> > > oooh, yes. That's much better - thanks. > > We should make sure that change gets into the man pages... > > First however, we should probably correct the man page! > stat.2 says: > > > NOTES > On Linux, lstat() will generally not trigger automounter > action, whereas stat() will (but see the description of > fstatat() AT_NO_AUTOMOUNT fag, above). > > which is wrong: lstat and stat treat automounts the same. > @Michael: do you recall why you inserted that text? The commit message > in commit 1ef5b2805471 ("stat.2: Cosmetic reworking of timestamp > discussion in NOTES") is not very helpful. That commit really was just cosmetic changes. The change that introduced the text was 82d2be3d9d66b7, based on a note from Peter Anvin: [[ > > Additionally, you may want to make a note in the stat/lstat man page tha t on > > Linux, lstat(2) will generally not trigger automounter action, whereas > > stat(2) will. > > I don't understand this last piece. Can you say some more. (I'm not > familiar with automounter details.) An automounter (either an explicit one, like autofs, or an implicit one, such as are used by AFS or NFSv4) is something that triggers a mount when something is touched. However, it's undesirable to automount, say, everyone's home directory just because someone opened up /home in their GUI browser or typed "ls -l /home". The early automounters simply didn't list the contents until you accessed it by name; this is still the case when you can't enumerate a mapping (say, all DNS names under /net). However, this is extremely inconvenient, too. The solution we ended up settling on is to create something that looks like a directory (i.e. reports S_IFDIR in stat()), but behaves somewhat like a symlink. In particular, when it is accessed in a way where a symlink would be dereferenced, the automount triggers and the directory is mounted. However, system calls which do *not* cause a symlink to be dereferenced, like lstat(), also do not cause the automounter to trigger. This means that "ls -l", or a GUI file browser, can see a list of directories without causing each one of them to be automounted. -hpa ]] Cheers, Michael > I propose correcting to > > NOTES: > On Linux, lstat() nor stat() act as though AT_NO_AUTOMOUNT was set > and will not trigger automounter action for direct automount > points, though they may (prior to 4.14) for indirect automount > points. > > > The more precise details, that automount action for indirect automount > points is not triggered when the 'browse' option is used, is probably > not necessary. > > Ian: if you agree with that text, and Michael doesn't provide alternate > evidence, I'll submit a formal patch for the man page.... or should we > just wait until the patch actually lands? > > Thanks, > NeilBrown > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
